1. Field of the Invention
The present invention relates to a channel controlling methods and apparatus for data service in a mobile communication system and, more particularly, to methods and apparatus for initializing a reverse packet traffic channel.
2. Description of the Related Art
FIG. 1 is a block diagram of a terminal (e.g., fixed or mobile subscriber station), a base station, and a radio link in a mobile communication system. The radio link of FIG. 1 is composed of a forward channel for data transmission from the base station to the terminal and a reverse channel for data transmission from the terminal to the base station.
A conventional CDMA mobile communication system has a forward channel structure as shown in FIG. 3 and a reverse channel structure as shown in FIG. 2. The forward CDMA channel has a pilot channel, a sync channel, a paging channel, and a forward traffic channel divided into a fundamental channel and a supplemental channel. The reverse CDMA channel includes an access channel and a reverse traffic channel divided into a fundamental channel and a supplemental channel.
A conventional bidirectional traffic channel has too low a bit rate to provide a packet data service. An approach to ensuring a bit rate high enough for implementing a packet data service in a conventional bidirectional traffic channel is to separate the traffic channel into a fundamental channel and a supplemental channel. The fundamental channel provides the same function of the conventional bidirectional traffic channel and the supplemental channel provides packet data communication. For a packet data service, a call is maintained between a base station and a terminal, as shown in FIG. 1, via forward and reverse fundamental channels.
FIG. 4 is a state transition diagram of a terminal on the basis of a conventional CDMA standard. Referring to FIG. 4, when power is on in a state 411, the terminal is set to an initialization state 412. If the terminal synchronizes its timing to that of a system in the initialization state 412, the terminal transits to an idle state 413. The terminal may attempt a call, the base station transmits information of the call attempt to the terminal on a paging channel, and the terminal sends a page response message to the base station, in the idle state 413. Then, the terminal is set to a system access state 414. If the terminal fails to obtain a paging channel message or is directed from the base station to a different adjacent base station in a handoff in the idle state 413, the terminal returns to the initialization state 412. Here, if the terminal succeeds in system access except for the call attempt or reception of a call acknowledge signal in the system access state 414, the terminal returns to the idle state 413. However, if the terminal succeeds in the call attempt or reception of the call acknowledge signal in the system access state 414, the terminal goes to a traffic channel state 414. The traffic channel state 414 lasts as long as traffic is processed. When traffic channels stop their action, the terminal returns to the initialization state 412.
A communication system maintains a call on bidirectional fundamental channels despite infrequent transmit/receive packets in the conventional packet data service. In this case, a signal transmitted on the bidirectional fundamental channels interferes with another channel, resulting in wasted capacity of another supportable service in the CDMA communication network. Furthermore, as more users demand the packet data service, the CDMA mobile communication network becomes less accessible to users confined to voice communication on fundamental channels only.
Packet data is transmitted according to the call state transition diagram of FIG. 4 in a CDMA system based on IS-95. Such call processing is suitable not for a connectionless oriented service, such as a packet data service, but for a connection oriented service, such as a voice service, because a large amount of time is required to reach mobile acquisition due to too many states involved between a call drop and a call set-up and transmission of many messages. As shown in FIG. 4, the initialization state 412, the idle state 413, and the system access state 414 should be passed through before resuming the traffic channel state 415.
Therefore, there is a need for simplifying the state transition for a call set-up by introducing a packet idle state and a packet busy state so that a reliable packet traffic service can be provided by use of mobile acquisition information only, from an initial call set-up to a call reset-up for a packet service.
Another problem associated with conventional packet data service is the mobile acquisition method. A mobile acquisition rate may be insignificant in a service having a call maintained to the end, such as a voice call, but it is a significant factor to determine system throughput in a service required to disconnect a call, such as a packet data service. Thus, the entire system performance would depend on how fast a terminal transits from the packet idle state to the packet busy state. In conventional mobile acquisition, power control starts when a base station sends an acknowledgement after acquisition of an 80 ms or longer access channel. If the base station fails to acquire the access channel and send an acknowledgement, an access probe is transmitted at a power level that is a base station-specified amount higher than the previous access probe after a 160 ms or longer delay designated by the base station due to impossibility of fast power control on the access channel.
The above-described conventional packet data service has, inter alia, the following problems:
(1) The 80 ms or above access channel is longer than needed in a packet data service.
(2) Both a terminal and a base station wait longer than needed because power control begins only after the acknowledgement of the base station. The delay may be one or more seconds, which is a long time in consideration of a CDMA frame being 20 ms in duration. Thus, possible fast power control at the start of access can reduce the time required for retransmission of an access probe after a delay.
(3) Despite a random delay for preventing access collision between terminals in CDMA access, access collisions increase with more users of a packet data service, which is a significant cause of access failures since another user""s signal acts as interference in CDMA.
An object of the present invention is to provide methods and apparatus for initializing a reverse packet traffic channel, which can reduce an access time during a packet data service.
Another object of the present invention is to provide methods and apparatus for initializing a reverse packet traffic channel, which can reduce an excessive output signal magnitude of a terminal by rapidly implementing power control at an initial access time point.
Still another object of the present invention is to provide methods and apparatus for initializing a reverse packet traffic channel, which can prevent access collisions by removing a random delay in an access to a packet data service in a mobile communication system.
To achieve the above and other objects, there is provided methods and apparatus for initializing a packet traffic channel in a communication system. In one method and apparatus, a delay time of a forward channel signal transmitted from a base station to the terminal is detected and reported to the delay time to the base station, and the timing of the terminal is synchronized to that of the base station, in a packet idle state after a call set-up. Access collision between terminals attempting to access to a base station is controlled by assigning a specific time slot to each terminal in an access attempt and causing the terminal to access in the assigned time slot. A mobile acquisition is rapidly achieved by repeatedly using short preamble patterns for terminals. A power control is performed on the packet traffic channel by adding correlation values in a search window of the specific time slot.